The short version (and maybe someone else can do this more succinctly) is that each piece of gear (Face, Sword, Knife, Gun, Drone, Hair, Whatever) has 3 different slots to unlock and you get a little bit of experience for them every single fight you're wearing them. You won't know what type of slot it is until its unlocked and its differnet for every single piece in the game. Once its unlocked (you can see them and your progress on the tiny little window that pops up after every fight) you can go into customize, go into the gear and hit square (or whatever) to bring up which augments you have that fit said slot.
Again, if you've been playing and you haven't changed your clothes a ton you've definitely got slots open. If you've been messing around in ToT or the Krypt alot you've definitely also unlocked some augments for your characters. On the customize character selection screen you can see (and check) how many augments you have for each character.